In refrigeration systems utilizing a holocarbon refrigerent fluid, as for example R-12, R-22, R-502, etc., pumped by a compressor through a condensor, an expansion valve and an evaporator and then returned to the intake side of the compressor, it is highly desirable to include a filter-drier unit for filtering particles of solid foreign matter from the fluid and for adsorbing water, acids, and other impurities from the fluid. Such foreign matter and impurities may be present at system startup or may be introduced at a later time because of motor burnout, leakage, in the suction line of the compressor, decomposition of organic substances in the compressor, etc. In the latter cases viscous tars are frequently formed that may plug capillary tubes, expansion valves and cause other damage unless removed.
Filter-drier units have heretofore been made in various forms, including solid but porous blocks through which the refrigerant fluid is required to pass. Such solid blocks have been made from granular adsorbent material such as zeolite molecular sieves, activated carbon, alumina, silica gel, or the like and wherein the granules have been adhered to each other by various binders. Heretofore such blocks when they contain sufficient binder have a comparatively high pressure drop because the binder material blocks pores between the granules.
Thus, for example, core blocks made in accordance with one current practice wherein adsorbents of 14 by 30 mesh grain size and constituting about 85% by weight of the block are bound together by a phenolic resin comprising the remaining 15% have a presure drop of about 11.7 inches of water through a section about 3/8" thick when air flows therethrough at about 400 cubic feet per hour. A pressure drop of this magnitude wastes energy during operation of the system.
Generally, most present commercially available filter driers use an inorganic binder material such as aluminum phosphate. These materials are relatively weak binders and therefore must be used in a relatively high proportion with respect to the adsorbents to yield a core block of sufficient tensile strength and resistance to spalling. Because of the high proportion of binder, the adsorbents must be of relatively large grain size to minimize clogging of the spaces between desiccant particles. The net result is a core block of relatively high pressure drop.
When organic binders are used the core blocks are generally formed by pressure molding wherein the binder-disiccant mixture is compacted in a mold between two punches. After compacting, the block is forced from the mold by one of the punches. This wipes the surface of the block and the binder flows into the clogs more of the pores near the surface than would otherwise occur. This also contributes to high pressure drop in the finished core block.